Polypropylene materials are frequently used for pipes for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid is pressurised and/or heated. In particular, polypropylene materials are used in applications for plumbing and heating, such as in-house hot and cold water pipes and fittings, floor and wall heating systems and radiator connections.
From EP1448631 it is known to use polypropylene which crystallises predominantly in the β-modification for pressure pipes having increased long-term pressure resistance. Polypropylene which crystallises predominantly in the β-modification has an increased impact strength compared to polypropylene when it is crystallised in the α-modification. Generally, the higher the amount of β-modification, the better the impact strength of the polypropylene will be.
With regard to polypropylene pipes, impact strength of pipes is often measured by the falling weight impact test at 0° C. according to EN 1411. For pipes this is a more practice-oriented parameter, since it correlates to the resistance of the pipe against breakage during installation, especially at low temperatures.
Further, it is customary to use coloured polypropylene for these applications. A number of organic and inorganic pigments exist, which are used for the production of coloured polypropylene. However, many organic pigments are unsuitable for the production of coloured polypropylene having a high content of β-modification, because organic pigments are often acting as strong α-nucleating agents. When both α-nucleating agents and β-nucleating agents are present in a polypropylene, the α-nucleating agent is usually predominant, because the α-modification is thermodynamically more favoured than the β-modification. Inorganic pigments are favoured for the abovementioned purposes.
One of the usual colours which is used for such materials is green. Usually, the colour green is achieved by the use of specific inorganic pigments, especially the pigments C.I. Pigment Green 17, C.I. Pigment Green 26 and C.I. Pigment Green 50 (c.f. Plastics Additives Handbook, 5th Edition, Edited by Dr. Hans Zweifel, Carl Hanser Verlag, Munich 2001, p. 849). Chemically, C.I. Pigment Green 17 is Cr2C3. It is also identified by CAS No. 68909-79-5. Chemically, C.I. Pigment Green 26 is CoCr2C4. It is also identified by CAS No. 68187-49-5. Chemically, C.I. Pigment Green 50 is (Co,Ni,Zn)2Tia4. It is also identified by CAS No. 68186-85-6.
It was however observed, that the use of the abovementioned pigments to produce a green coloured β-nucleated polypropylene composition has the disadvantage that the impact strength (measured as falling weight impact test) is still unsatisfactory.